Transcript Lecture02
Astrophysics from Space Lecture 2: Beating the atmosphere Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year 2014-2015 The Earth atmosphere Lord Rayleigh (1842-1919) The Earth atmosphere Relative concentrations of permanent gases in the atmosphere is relatively constant. Non-permanent gases: H20 and O3 Water vapor in the atmosphere H20 (and CO2 to some extent) are responsible for most of the extinction in the optical and NIR regime Good news: H20 is low-altitude phenomenon Mauna Kea – Hawaii La Silla – Chile Paranal – Chile Optical observatories Most large ground-based optical/NIR observatories are located on mountain tops Diffraction of light Diffraction is a natural consequence of the wave nature of light. Airy diffraction pattern Diffraction of light Airy diffraction: point sources are converted to a disc (Airy disc) with size θmin ~ 1.22 λ/D Example: VLT with D = 8.2 m optical radiation at λ = 550 nm θmin = 0.017 arcsec Unfortunately, it is currently impossible to realize diffraction limited observations in the optical, due to optical aberrations in the telescope and atmospheric turbulence. Active optics Elimination of the aberrations in the telescope optics by (continuous) correction of the telescope shape. Result: seeing-limited observations Atmospheric turbulence Atmospheric turbulence spreads out optical radiation to a much bigger disk (seeing disc) than the diffraction limit. The seeing depends critically on the site. Typical seeing values: Belgium: few arcsec La Palma: 1 arcsec Paranal: 0.6 arcsec Dome C: 0.3 arcsec Atmospheric turbulence = In seeing-limited observing, the 8 m VLT telescope has the same resolution as an amateur 30 cm telescope Beating the atmosphere Seeing is the result of “speckles” moving on the plane of the sky at atmospheric time scales 10 ms to 100 ms Speckle imaging and Lucky imaging Use very fast, short-exposure images and combine them at the end of the observation • speckle imaging: combine all images • lucky imaging: select only the “best” images Traditional image Exposure time 300s Speckle image Exposure time 300s Lucky image Exposure time 15s More Lucky imaging examples Cat’s Eye nebula Globular cluster M13 Adaptive optics 1. Measure the distortions in the wavefront 2. Compensate for them using deformable mirror or liquid crystal array Adaptive optics Advantage of AO: one can keep integrating (so not limited to bright sources as speckle/Lucky imaging) Problem: bright reference star necessary: NGS/LGS Adaptive optics Adaptive optics Still many problems • reliable LGS AO systems are still rare… • atmospheric conditions must be optimal for AO • turbulence cells are typically smaller than the telescope -> multi-conjugate AO The solution: space telescopes